This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application Nos. 2000-354116, 2001-4983, and 2001-17149, filed in Japan on Nov. 21, 2000, Jan. 12, 2001, and Jan. 25, 2001, respectively, which are herein incorporated by reference.
1. Field of the Invention
The present invention generally relates to a variable valve timing apparatus for adjusting opening and closing timing of an intake valve and an exhaust valve in an external combustion engine (hereinafter, referred to as xe2x80x9cenginexe2x80x9d)
2. Description of Related Art
It is known that an overlap in the opening time of an exhaust valve and an intake valve is increased at the cold start of an engine to reduce the emission of unburned HC. For example, Japanese Patent Laid-open Publication No. 11-336574 discloses closing an exhaust valve usually at a top dead center (TDC) of the intake stroke, advancing the exhaust valve to improve the afterburning effect in cold starting, and advancing an intake valve by the maximum amount to increase an overlap to thus increase the internal EGR. The internal EGR is the gas that is exhausted to the intake side when an intake valve opens in an exhaust stroke, and reenters a cylinder in the next intake stroke.
According to the prior art disclosed in the above publication, however, if there is liquid fuel, a part thereof is exhausted without undergoing a combustion stroke since the overlap lies ahead of the TDC, that is, in an exhaust stroke.
If an intake port injection type engine is given as an example, fuel injected into an intake port adheres to the side (back side) of an intake valve away from a combustion chamber and to the intake port just after the cold start of the engine, and is stored in the form of liquid in the vicinity of a lower valve sheet due to the tare weight thereof while an intake valve is opened. If the intake valve is opened in the exhaust stroke (if an overlap lies in the exhaust stroke), the fuel flows directly into a cylinder during the first stroke at clanking (the first opening of the intake valve). Although the exhaust gas in each cylinder flows back into an exhaust pipe after the first stroke, the fuel partially flows into the cylinder due to the tare weight since the fuel is in the form of a liquid.
The fuel is exhausted directly to an exhaust side by the upward movement of a piston, or is evaporated and partially discharged in the form of unburned fuel to an exhaust side. Then, the exhaust valve closes before the TDC to inhibit the unburned fuel, having passed through the cylinder, from returning into the cylinder, or the unburned fuel is discharged directly into the atmosphere without after-burning due to the low temperature of the fuel. If the engine temperature is then increased by repeated combustions, the fuel is atomized due to the heated intake port, as a result of hot exhaust gas blowing into the intake port, to inhibit the liquid fuel from flowing into the cylinder and entering an exhaust passage.
Therefore, in order to reduce the emission of unburned HC at the cold start of the engine, it is necessary to prevent the discharge of liquid fuel, which cannot be atomized just after the cold start, before increasing the internal EGR to facilitate the atomization of the fuel.
It is therefore an object of the present invention to provide a variable valve timing apparatus capable of properly controlling an overlap in the opening time of an intake valve and an exhaust valve to surely control the emission of unburned HC at the cold start of an engine.
The above object can be accomplished by providing a variable valve timing apparatus that increases an overlap in opening time of an intake valve and an exhaust valve at cold start of an internal combustion engine, wherein the overlap comprises an exhaust stroke range ahead of a top dead center and an intake stroke range after the top dead center; and there is provided a valve timing control means for forming an overlap including the intake stroke range just after the internal combustion engine is started at the cold start, and then increases the overlap in the exhaust stroke range.
Therefore, at the cold start of the engine, the overlap in the opening time of the intake valve and the exhaust valve is controlled to include the intake stroke range just after the cold start and to then increase the exhaust stroke range. At the cold start, when the fuel is not facilitated to atomize, the fuel injected into an intake port is stored in the form of liquid in the vicinity of a valve sheet while the valve is opened, but as a piston moves downward during the overlap in the intake stroke range just after the cold start, the liquid fuel flows into a cylinder without being directly discharged so that the fuel can be combusted without fail. If the overlap in the exhaust stroke range is then increased, exhaust gases or the like having exhausted once to the exhaust side flow back into the intake port to prevent the discharge of the liquid fuel, or to prevent an after-burning effect resulting from the early opening of the exhaust valve raises the temperature of a catalyst.